Advances in the telecommunications industry have caused improvements in virtually every aspect of telecommunication links. For example, fiber optic technology has enjoyed rapid advancement and implementation, and appears to provide benefits which will be realized into the next century. Digitization of communications permits increased performance at lower costs using integrated circuits. Less noise, lower signal to noise ratio requirements, and lower error rates are additional advantages. Another example, and one which often uses fiber optic technology, is the implementation of digital telecommunication rings. In these rings, communication occurs between various network elements along the ring. These rings are beneficial because rings of virtually any length are practical. Moreover, rings add reliability to the communication between the various network elements along the ring.
With the advancement of communication rings, various regulatory agencies and specifications are developing. For example, as is known in the telecommunications art, the synchronous optical network (SONET) is the new ANSI standard for advanced fiber optic transmission. SONET has for the first time defined a standard optical interface which allows so-called "mid-span" meets, that is, interfaces between equipment produced by different manufacturers. This standard has particular application to the present invention in that it describes common generic criteria for optical ring networks.
As is known in the art, network elements (NEs) communicate around a ring by passage of information frames. Under SONET, entire communication streams do not have to be torn apart and reassembled every time a signal is added or dropped in a SONET network. Instead, they are collected and routed within a level 1 synchronous transport signal (STS-1) frame. FIG. 1 illustrates an STS-1 frame. The STS-1 frame consists of 90 columns and 9 rows of 8-bit bytes (shown as "B"), for a total of 810 bytes (6480 bits). Typically, the STS-1 frame has a length of 125 microseconds (i.e., 8,000 frames per second). The bytes of the STS-1 frame are transmitted in a row-by-row fashion from right to left. Further, for each byte, the most significant bit is transmitted first. Note also that multiple STS-1s may be synchronously multiplexed into higher rate STS-N signals. STS-N signals are converted to optical OC-N signals for transport through fiber optic media.
The first three columns of the STS-1 frame are designated the transport overhead. The transport overhead contains overhead bytes of both section overhead and line overhead. Under current standards, twenty-seven bytes are assigned for transport overhead, with nine bytes of section overhead and eighteen bytes of line overhead. The section overhead deals with the transportation of an STS-N frame across the physical layer or physical media of the ring. Functions of this section overhead include framing, scrambling, section error monitoring, and communicating section level overhead. The line layer provides synchronization and multiplexing functions for the path layer (the path layer deals with the transport of network services between SONET terminal multiplexing equipment). The line overhead associated with these functions includes overhead for maintenance and protection purposes.
As also known in the art, SONET functionally specifies particular bytes in the section and line transport overhead. These bytes are also referred to in the art as defining "channels". FIG. 2 generally illustrates these bytes with the letter designations given by SONET. While the position of each byte is specified by SONET, the functionality of various bits or even complete bytes for particular designations remain unspecified for certain applications. For example, and as set forth in greater detail below, the K1 line overhead byte has no current specification for unidirectional rings. As another example, bits 3-5 (with bit 1 being the most significant bit) of the K2 line overhead byte are likewise unspecified by SONET for unidirectional rings. For other definitions and descriptions of the transport overhead bytes, the reader is referred to Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria, Technical Reference TR-NWT-000253, Issue 2, December 1991, published by Bellcore, and incorporated herein by reference.
Returning to FIG. 1, the remaining 87 columns of the STS-1 frame are the STS-1 envelope capacity. Within the STS-1 envelope capacity is placed a synchronous payload envelope (SPE). One column of the SPE contains nine bytes, designated as STS path overhead. This column may be located at any column within the STS-1 envelope capacity. The remaining 774 bytes are available for payload. The STS-1 SPE may begin anywhere in the STS envelope capacity. Typically, it begins in one frame and ends in the next. The STS-1 SPE may, however, be wholly contained in one frame. STS path overhead is associated with each payload and is used to communicate functions from the point where service is mapped into the STS SPE to where it is delivered.
FIG. 3 illustrates a simplified block diagram to delineate various definitions of the transmission network that interconnect various SONET NEs. Specifically, the SONET line, section and path are shown. FIG. 3 further illustrates path terminating equipment 10 and 12, line terminating equipment 14 and 16, and section "terminating" equipment 18 and 20. As known in the art, an NE is said to be terminating if it is specified that during normal operations, the device may alter the corresponding information associated with it. For example, path terminating equipment 10 and 12 are defined to be equipment which are permitted to alter the path portion of the SONET frame (see FIG. 1). Line terminating equipment 14 and 16 are specified so that they are permitted to alter the line information of the SONET frame. Finally, section terminating equipment 18 and 20 are specified so that they may alter the section information of the SONET frame.
An example of STS-1 SPE path terminating equipment is an add/drop multiplexer. An add/drop multiplexer is also line terminating. Finally, an example of section terminating equipment is a repeater. Note that a device specified as capable of terminating one category of information necessarily can terminate subset categories. Thus, path terminating equipment 10 and 12 may also terminate line and section information. Further, line terminating equipment 14 and 16 may also terminate section information. Note also that the termination specifications apply only during normal operations. For example, while a repeater is section terminating, during a failure it may alter other information such as the K1 and K2 line overhead bytes.
Given the terms and standards set forth above, note that currently no specification exists for restoring the transport overhead information (outside of the DCC) in a SONET ring network. Given the broad base possibility for incompatibilities at a mid-span interface, it is important to develop a standard method for control of the frame transport overhead. Specifically, it is highly beneficial to develop a system whereby transport overhead is accounted for during a failure along the ring network. By maintaining part or all of the transport overhead operational during a system failure, it is possible to use the transport overhead to help identify the failure for purposes of having it corrected in an expedited manner. Moreover, once the network is repaired, it is also important to restore the transport overhead in an organized and efficient manner.
As known in the art, improper restoration of the transport overhead channel in the ring network may cause an oscillatory action to occur along the network. Such oscillation may occur when the ring becomes "closed" as to transport, that is, having no barrier to prevent the transport overhead from continuously encircling the ring. This unimpeded travel allows an NE to add transport overhead to the ring, and subsequently receive back the same overhead, in addition to any accumulated overhead added by other NEs. This cumulative feedback around the ring may provide undesirable results. For example, for audio transport overhead, such undesirable effects may include high level audio feedback which is received by any NE monitoring the ring.
FIG. 4a illustrates a block diagram of a prior art telecommunications ring network 22. According to this example, ring network 22 is a unidirectional path protection switched (PPS) telecommunications ring network. A unidirectional ring, as known in the art, is one in which traffic generally travels in one medium and direction between elements along the ring, while concurrently flowing in a redundant manner in an opposite direction in another medium. Note also that transport overhead, as opposed to traffic, is communicated bidirectionally between NEs on a unidirectional ring. Ring network 22 includes a first ring 24 and a second ring 26. As illustrated, traffic flow around first ring 24 is clockwise, while traffic flow around second ring 26 is counterclockwise. Further, both rings 24 and 26 traverse through various NEs positioned along the ring. In the example of FIG. 3a, four NEs are provided and are designated with corresponding numerals (e.g., NE0, NE1, NE2 and NE3).
Typically, one of rings 24 or 26 is predetermined as the primary communication medium for traffic flow around the ring. For example, consider first ring 24 as this predetermined ring. As a result, communication between any of the NEs along the ring is, under normal operating conditions, in a clockwise fashion. For each communication, however, a redundant signal is provided along second ring 26 in a counterclockwise fashion. As is known in the art, this redundant signal permits the network element to select between the primary and redundant signals according to the transport performance information received by the corresponding NE.
FIG. 4b illustrates network ring 22 of FIG. 4a, with the additional indication that NE0 has implemented an "artificial transport overhead break" 28. Such an implementation of a transport overhead break is known in the art for preventing ring network 22 from becoming a closed ring. Without overhead break 28, transport overhead would be permitted to continue uninterrupted around the entirety of ring 22 and, hence, could cause the undesirable accumulation and feedback effects discussed above. The inclusion of transport overhead break 28, however, acts as a barrier so that the transport overhead is not accepted from what is illustrated as the right side of NE0. Thus, the effect of the overhead break 28 is to prevent the transport overhead from fully encircling the ring and, consequently, also to prevent undesirable cumulative feedback effects.
FIG. 4c illustrates a block diagram of the effective communication path for the transport overhead caused by the artificial transport overhead break 28 illustrated in FIG. 4b. As shown, the direct line of transport overhead communication between NE0 and NE3 is effectively severed by artificial transport overhead break 28. As a result, any transport overhead communication between these two NEs must be communicated through NE1 and NE2. Nonetheless, transport overhead communication is still permitted between each NE of the ring, without the possibility of cumulative feedback.
FIG. 4d illustrates ring network 22 of FIG. 4a, wherein both rings 24 and 26 have been severed (illustrated by an "X" on rings 24 and 26). The severance may occur due to a physical disturbance of rings 24 and 26, or like situation. In any instance, as known in the art, the NEs along the rings include circuitry for detecting the loss of an incoming signal. When a signal loss occurs, a detecting NE inserts an "actual" transport overhead break on the side of the NE which would receive the signal if it existed. An actual overhead break is to be contrasted with an artificial overhead break. The former occurs upon detection of an actual fault with the ring or one of its components. The latter is imposed, as discussed above, to create a break in transport overhead communications during normal operations of the ring network. Note also that, during an artificial break, the NE imposing the break may evaluate the transport overhead. It does not, however, transmit the overhead to the using application, or pass it through to the remainder of the network.
As an example of an actual overhead break, ring network 22 is shown with an actual failure between NE2 and NE3. In response to the failure, NE2 and NE3 detect a loss of signal along rings 26 and 24, respectively. Upon detection of this signal loss, both NE2 and NE3 force an actual transport overhead break 30 and 32, respectively, on their corresponding sides detecting the failure. Overhead breaks 30 and 32 function in the same manner as artificial overhead break 28 associated with NE0. Thus, breaks 30 and 32 preclude transport overhead information received by NE2 from transmitting along ring 24 toward NE3. Similarly, overhead break 32 prevents overhead information received by NE3 from transmitting along ring 26 toward NE2.
FIG. 4e illustrates the effective communication path for the transport overhead of network ring 22. Note that overhead breaks 28, 30 and 32 act in combination to segment or isolate NE3 from the remainder of the NEs of ring 22. Thus, before restoring the ring or making any provision for this condition, NE3 cannot communicate transport overhead with the remainder of the ring network. Clearly, such a result is undesirable because the lack of overhead communication to NE3 prevents using that information to help troubleshoot the fault which has occurred along the ring network. In addition, no current standards exist for handling the transport overhead channels in a SONET ring upon the imposition of an actual overhead break. Indeed, in some prior art systems, the ring network is simply left in its segmented form, with no restoration of transport overhead to the segmented NE or NEs. One key object of the present invention, as more readily appreciated below, is to maintain a constant communication path between all NEs despite a fault along one of the rings (for example, as shown in FIG. 4c).
One known solution for attempting to restore transport overhead is implemented in the LTS-21130 ring network, formerly owned and sold by Rockwell and currently owned by Alcatel. The LTS-21130, however, is dependent on a strict hardware implementation. In this implementation, a "home node" initially imposes an artificial transport overhead break. Thereafter, an NE along the ring, when detecting a loss of signal, inserts a transport overhead break in the direction of the loss of signal. Upon correction of the break, the detecting NE immediately removes its overhead break. In addition, the detecting node transmits an indication bit and the newly received signal toward the home node. The home node, upon receiving the forwarded information, reinserts its overhead break. Prior to receiving this new information, however, the ring is a closed ring, that is, no overhead break exists on the ring and, therefore, cumulative overhead feedback may occur. Indeed, specific dedicated hardware is included in the home node so that it can quickly reinsert its overhead break before the effects of cumulative overhead feedback become overwhelming.
Thus, in the LTS-21130, additional specific hardware is necessary for quick switching so that the home node can insert its overhead break prior to permitting an immense amount of cumulative feedback to occur. Further, this restoration process is not predictable because the speed of the restoration relies strictly on the speed of the hardware. In contemporary networks, however, software, rather than hardware, is commonly used to manage the network. Moreover, the primary consideration during a failure along the ring is to restore traffic, rather than overhead. Such software restoration processes are well known in the art. Thus, in a software based environment, the scheme of the LTS-21130 is impractical because: (1) the use of dedicated hardware is undesirable; and (2) the speed required to implement the scheme is unavailable because the software is initially appointed to reestablishing traffic, rather than overhead around the ring. In contrast, the present invention provides a deterministic (i.e., predictable and uniform in result) method in which the reinsertion of an overhead break is ensured to occur prior to the release of the break elsewhere in the ring. Thus, the ring is never fully closed to overhead, thereby preventing cumulative feedback from occurring. Further, the present invention is preferably embodied in software, rather than dedicated hardware.
It is therefore an object of the invention to provide a method and system for restoring some or all of the transport overhead channels in a SONET ring.
It is a further object of the present invention to provide such a method and system which is useful in both SONET unidirectional and bidirectional rings.
It is a further object of the present invention to provide such a method and system to prevent oscillatory action around the network ring due to cumulative feedback of the transport overhead channels.
It is a further object of the present invention to provide such a method and system for providing a deterministic method to restore the overhead channels of a network ring.
It is a further object of the present invention to provide such a method and system such that there are no requirements as to how the NEs are distributed in the network ring while still having the ability to detect and restore the network transport overhead channels.
It is a further object of the present invention to provide such a method and system which does not require knowledge of the network topology other than the type of ring operation (i.e., unidirectional or bidirectional) and which path terminating device is the ring master.
It is a further object of the present invention to provide such a method and system such that overhead operations do not interfere with or delay traffic protection.
Still other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.